A fuel cell refers to a battery directly changing chemical energy generated by oxidation of a fuel into electric energy (that is, electricity) and has high energy efficiency and eco-friendly features such as reduced discharge of contaminants, thereby becoming popular as a next-generation energy source.
The fuel cell generally has a structure comprised of an oxidation electrode (anode) and a reduction electrode (cathode) by interposing an electrolyte membrane, and such a structure is referred to as a membrane electrode assembly (MEA).
The fuel cell may include an alkali electrolyte membrane fuel cell, a polymer electrolyte membrane fuel cell (PEMFC), etc., in terms of types of electrolytes, and among these, the polymer electrolyte membrane fuel cell shows advantageous features such as low operational temperature of less than 100° C., rapid start and response characteristics, excellent durability, etc. and is thus receiving considerable attention as a power device for portable use, automobiles and/or household appliances.
As a representative example of such a polymer electrolyte fuel cell, there is a proton exchange membrane fuel cell (PEMFC) using hydrogen gas as a fuel.
A reaction occurring in the PEMFC may be briefly described as follow. First, since a fuel such as hydrogen gas is supplied to an oxidation electrode (hereinafter, anode'), protons (H+) and electrons (e−) are generated on the anode by oxidation of hydrogen. The generated protons (H+) are delivered to a reduction electrode (hereinafter, ‘cathode’) through a polymer electrolyte membrane, while the generated electrons (e−) migrate to the cathode via an external circuit. The cathode may be provided with oxygen and the oxygen may be combined with protons (H+) and electrons (e−) to produce water by reduction of the oxygen.
The polymer electrolyte membrane is a passage through which the proton (H+) is transferred to the cathode, which therefore, must have a high proton (H+) conductivity. In addition, the polymer electrolyte membrane should have excellent separation ability that separates hydrogen gas supplied to the anode and oxygen fed to the cathode, respectively. Moreover, the polymer electrolyte membrane needs superior mechanical strength, dimensional stability, chemical resistance, etc. while having reduced ohmic loss at a high current density, in addition to the foregoing characteristics.
The polymer electrolyte membrane currently used in the art may include, for example, a fluorine resin such as perfluorosulfonic acid resin (hereinafter, referred to as a ‘fluorine ionomer’). However, the fluorine ionomer has a low mechanical strength and, when used for a long period of time, pinholes may be generated which in turn causes deterioration in energy conversion efficiency. Although increasing a membrane thickness of the fluorine ionomer has been attempted to reinforce mechanical strength, this increases not only ohmic loss but also consumption of an expensive raw material, thus being less economical.
In order to solve the foregoing problems, a polymer electrolyte membrane having improved mechanical strength prepared by impregnating a porous polytetrafluoroethylene resin (trade name: Teflon) as a fluorine resin (hereinafter, referred to as a ‘Teflon resin’) with a fluorine ionomer in a liquid status has been proposed.
In this case, compared to a polymer electrolyte film comprised of a fluorine ionomer alone, the above prepared membrane has relatively high mechanical strength, thus decreasing a thickness of the electrolyte membrane and finally attaining advantages such as a decrease in ohmic loss, even though proton conductivity may be slightly reduced.
However, since the Teflon resin has very low adhesiveness, selection of ionomers is restricted. In addition, a fluorine ionomer-applied product entails a disadvantage of high crossover (phenomenon), compared to hydrocarbon based products. Further, since the porous Teflon resin as well as the fluorine ionomer are expensive, development of cheap and novel materials for mass-production is still required.